Significant advances have been made in the miniaturizaton of electronic components, resulting in reduced component cost as well as size. Modern integrated circuit technology allows very small but complex circuits to be formed using mass production techniques on silicon and other substrates. Consequently, for applications such as pressure and force sensing, the transducers required to interface the electronic circuitry with the ambient environment typically occupy much greater volume and are much more costly than the electronic components used to process the signal from the transducers.
Displacement transducers are three-dimensional electro-mechanical structures which electronically monitor geometric deformation to measure applied forces. Pressure transducers are displacement transducers in which the displacement is caused by a differential in pressure across a deformable barrier, adding the requirement that a reference pressure be maintained on one side of the barrier. Present commercial pressure transducers are relatively large, discrete devices, typically formed utilizing a metal diaphragm as the pressure barrier. To minimize cost and size of components, as well as to reduce the packaging and fabrication costs of processing circuitry, it would be desirable to incorporate the pressure tranducer, or multiple transducers, directly on or in the substrate on which the electronic processing circuitry is formed. However, the physical structure required for a pressure transducer is not easily realized using conventional integrated circuit processing techniques.
It is possible to form pressure transducers in silicon substrates by selective etching of the substrate until the etch is terminated by a highly doped layer to form a thin membrane, and thereafter bonding another member over the membrane to define a cavity. Examples of such structures are shown in the patent to Guckel, et al., 4,203,128. While such structures are useful, their production requires processing steps not utilized in ordinary integrated circuit processing in which the electronic components are formed on only one side of the substrate. The heavy doping of one surface of the substrate required to provide the etch stop layer also imposes limitations on the ability to form electronic components on the doped surface using conventional deposition techniques.